1. Field of the Invention
This invention relates to a photodiode module and a method of making a photodiode module. The module has a photodiode and an optical fiber and is suitable for optical communication systems and optical measurements. The photodiode (PD) module is immune from distortion of signals and is capable of receiving various analog signals without distortion.
2. Description of Related Art
This application claims the priority of Japanese Patent Application No. 9-35566 (35566/97) filed Feb. 3, 1997 which is incorporated herein by reference.
As used herein, the term "photodiode module" refers to an assembly of a photodiode, an optical fiber, optical parts and a package. There are some proposals which deviate the PD chip axially forward or backward from the spot of the image of an end of the fiber by a lens in an assembly of a fiber, a lens and a PD chip. For example, Japanese Patent Laying Open No. 64-79629 (79629/'89) and Japanese Patent Laying Open No. 5-224101(224101/'93) suggest a photodiode module which displaces the relative position of the PD chip forward(inward) or backward (outward) from the image of the fiber using a lens for the sake of reducing the power of reflected light, decreasing the distortion of signals, etc.
The photodiode is a device having a package and a PD chip mounted in the package. The PD chip is a semiconductor chip having a pn-junction, an n-electrode and a p-electrode. One electrode through which light beams enter the PD chip is an annular electrode. FIG. 1 shows a section of a conventional, typical PD chip. The material of the light receiving layer is chosen in accordance with the wavelength of the light. Long wavelength bands, for example, 1.3 .mu.m band or 1.55 .mu.m band require a pin-photodiode having a light receiving layer of InGaAs mixture crystal. InGaAs photodiodes are produced on an epitaxial wafer having an n-InP substrate 81, an n-InP buffer layer 82, an InGaAs light receiving layer 83 and an n-InP window layer 84 grown on the InP substrate 81 in series in this order.
Selective diffusion of zinc (Zn) through a mask on the epitaxial wafer makes Zn-diffusing p-regions 85 and pn-junctions in the InGaAs light receiving layer 83. A p-electrode 86, an n-electrode 90, a passivation film 88 and an antireflection film 87 per a chip are formed on the wafer. The p-electrode 86 on the p-region 85 is a ring-shaped electrode having a wide opening through which incident light 89 enters the p-region 85. The n-electrode 90 uniformly covers the bottom of the n-InP substrate 81. The passivation film 88 protects the ends of the pn-junction, covering the outside of the ring p-electrode 86. The antireflection film 87 covers the p-region 85 for preventing the incident light from being reflected on the surface of the p-region region 85. The wafer is scribed into a plurality of individual photodiode (PD) chips.
An independent photodiode is made by building such a photodiode chip in a sealed metal-can package with a window. Otherwise, a PD chip is attached to an optical fiber or an optical connector for detecting optical signals propagating in the fiber. Such a device combining a PD chip via a lens with an optical fiber or an optical connector is called a "photodiode module (PD module)". In a PD module, an important matter is matching the fiber with the PD chip, since the PD must detect narrow beams emitted from a thin fiber. "Alignment" signifies an operation of matching a fiber to a PD chip in a PD module for maximizing the light power entering the PD chip via a lens and for providing maximum sensitivity to the PD module. Thus, alignment is one of the most important problems in building PD modules.
FIG. 2 shows a conventional photodiode module of a pig-tail type coupling an optical fiber to a PD chip. A PD chip 1 is fixed on a submount 13 soldered on a package 12. In coupling the PD chip 1 to the package 12, care should be taken for the alignment that the center of the PD chip 1 coincides with the center of the package 12. The center of a fiber 14 is positioned on an extension of the center axis line of the PD chip 1 and the package 12. All optical parts lie on a common axis in the prior PD module. This simple alignment is referred to as "common axis alignment". The package 12 has an anode pin 15, a cathode pin 16 and a case pin 17 extending downward from the bottom.
A cap 22 having a ball lens 23 is fixed on the package 12. The inner space of the package is hermetically sealed by the cap 22. A cylindrical sleeve 18 encloses the header of the package 12. Double-cylindrical ferrule holder 19 is welded on the upper surface of the sleeve 18. The ferrule holder 19 keeps a ferrule 20 which sustains an end of the optical fiber 14 along an axial line. A conical bend limiter 21 is fitted on the ferrule holder 19. An elastic material bend limiter 21 protects the end of the fiber from excess bending.
The PD chip 1 is mounted on the insulating submount 13 for separating electrically an n-electrode (cathode) of the PD from the package (case or ground) 12. The submount 13 is a rectangular insulator board made from aluminum nitride AlN, alumina Al.sub.2 O.sub.3 or the like. The submount 13 is coated on both surfaces with thin metal layers (called a metallized layer) for the convenience of soldering. The PD chip 1 is fixed on the submount 13 with e.g. PbSn solder. The upper metal layer of the submount 13 is joined to the cathode pin 16 with a gold wire. The p-electrode of the PD chip 1 is connected to the anode pin 15 with another gold wire.
The ball lens 23 ensures high efficiency entrance of light into the PD by converging the light beams transmitted from the optical fiber 14 onto the receiving surface of the PD chip 1. The end of the fiber 14 is slantingly polished with the ferrule 20 to prevent light reflected onto the fiber end from propagating back a laser which is positioned at the other end of the fiber. The slanting polishing angle is, for example, eight degrees. The slanting angle is an arbitrary parameter and is a matter of design choice. This PD module aligns all the centers of the fiber 14, the lens 23 and the PD chip 1 on the same center axis. This is the meaning of the "common axis alignment".
It seems as a matter of course that all the center axes of the optical parts coincide with each other in PD modules. The Inventors, however, have found that such a common axis alignment PD has a drawback of having a tendency of inducing interference between different frequencies in analog signal transmission. The drawback will be explained in detail. Optical CATV (cable television) is the most suitable example, because the mutual interference appears clearly in optical CATV PD modules. When an optical CATV system sends analog signals having many different frequencies to receiving sets containing PD modules, the received signals at the PD modules are sometimes distorted by the interference between signals having different frequencies. The number of channels is significantly restricted in order to avoid signal distortion in analog CATVs. Distortion occurs as follows. An optical CATV changes analog electric signals into analog optical signals by a laser (LD) or an LED and sends a plurality of light signals including different frequencies in an optical fiber to receiving sets of subscribers. The PD in the receiving set converts the optical signals to electric signals.
A unit using one frequency is called a "channel". An analog CATV station gathers the signals from a plurality of channels with different frequencies, converts the electric signals of the channels by a laser or an LED to optical signals including many analog signals and transmits the integrated signals through a fiber. The receiving set at a subscriber converts the optical signals into electric signals including many frequencies and selects one channel from the signals containing a plurality of channels. Since an analog PD module utilized analog signals, the PD must be highly linear. Linearity is one of the important criteria of analog PDs unlike digital PDs. Good linearity means that the electric signal (photocurrent I) is continually in proportion to the light power P over a wide power range in a photodiode. If the electric signal has higher order (nonlinear) terms of the light power, the signals are distorted by the interference between different frequencies in the PD. If the electric signal includes, e.g., a second order (quadratic) term of the original signals, new parasitic frequencies corresponding to a sum or a difference (beat) of two different frequencies appear in the PD output. This is the cause of signal distortion.
Distortion is a nonlinear phenomenon which appears also in all order harmonics higher than second order. The second order distortion IMD.sub.2 is the largest in value and the easiest to measure among all the distortions. Usually the performance of a PD is estimated by the second order distortion IMD.sub.2. The second order distortion is in proportion to the input signals. When the input signal is reduced, the IMD.sub.2 is also reduced. But reduction of the input signal leads to a low S/N ratio, which prevents a TV from displaying clear images. It is difficult for conventional PD modules to operate with low distortion without sacrificing signal power.
In an effort to solve the distortion problem, the Inventors once proposed a new analog PD module which can lower distortion by exploiting the aberration of a ball lens positively. This proposal is set forth in Japanese Patent Application No. 6-171873(171873/'94) "Analog PD module and method of producing same". FIG. 3 is a schematic diagram of optical parts in an analog PD module. A PD chip 1 is fixed on a package 2 via submount. The PD chip 1, a ball lens 3 and an optical fiber 5 align in series on a straight line. The fiber end is clumped by a ferrule 4. The parts or the structure are similar to a conventional PD module. The Inventors had first noticed a possibility of reducing the distortion without sacrificing sensitivity by varying the distance Z between the lens and the end of the optical fiber. Finding out the difference of the dependence upon distance Z between the distortion and the sensitivity, the Inventors succeeded in proposing a new arrangement of the PD, the lens and the fiber for making good use of the aberration of a ball lens.
FIG. 4 is a graph showing the AC sensitivity RAC (A/W) and the second order distortion IMD.sub.2 (dBc) measured by the Inventor as functions of the distance Z (mm) between the lens and the fiber. The abscissa is the distance Z (mm). The left ordinate is the AC sensitivity RAC (A/W). The right ordinate is the second order distortion IMD.sub.2 (dBc). The solid line shows the sensitivity RAC. The dotted line denotes the distortion IMD.sub.2. IMD.sub.2 has a peak value of -61 dBc at Z=1.2 mm. Conventional modules used to pay attention to only a larger distance Z far beyond the maximum distortion point (Z=1.2 mm) for seeking low distortion. A criterion of an analog PD requests an IMD.sub.2 lower than -75 dBc. The conventional PD module had adopted a large distance which first gives -75 dBc to IMD.sub.2 beyond the distortion maximum point. In the example, the prior PD module disposed the fiber end at a far point distanced by Z=1.6 mm from the lens (RAC=0.89 A/W). The point of the fiber, however, was diverted far from the maximum sensitivity range (RAC=0.96 A/W). The conventional PD module succeeded in reducing the IMD.sub.2 below -75 dBc at the expense of the sensitivity. The received signals were so weak that the PD module could not properly process the signals including many channels of different frequencies. The poor sensitivity had restricted the number of channels.
The Inventors considered the possibility of reducing only the distortion without decreasing the sensitivity of PD modules and measured the IMD.sub.2 and the RAC as a function of Z in a small Z region which has attracted no attention before. For the first time, the Inventors have noticed the existence of the range of a small Z which reduces the distortion faster than the sensitivity. In FIG. 4, there is a point giving IMD.sub.2 =-75 dBc between Z=0.8 mm and Z=0.9 mm which are far smaller than the conventional lens.multidot.fiber distance, e.g., Z=1.6 mm. Unlike the prior point Z=1.6 mm, the newly-found point between Z=0.8 mm and Z=0.9 mm can satisfy the requirements of both sensitivity and distortion. Then Japanese Patent Application No. 6-171873 gave a PD module maintaining high sensitivity and low distortion by determining the lens.multidot.fiber distance Z from Z=0.8 mm to Z=0.9 mm.
Why does the range of the maximum sensitivity broadly extend between Z=0.8 mm and Z=1.3 mm? The reason may be assumed as follows. The light receiving region enclosed by the p-electrode of a conventional PD chip has a wide area of a diameter W from 100 .mu.m (0.1 mm) to 200 .mu.m(0.2 mm). The ball lens converges the beams emitted from the fiber to a small spot (image of fiber end: beam waist) of a diameter U far smaller than 0.1 mm to 0.2 mm (U&lt;W). The beams converge on the PD to a (fiber end image) spot narrower than the light receiving region of the PD. Although the distance between the PD and the lens is maintained constant, the diameter of the beams on the PD varies as the fiber end moves in Z-direction. When the fiber lies at a point which gives the smallest diameter to the beam spot (beam waist) on the PD, all the beams enter the light receiving region. The fiber position realizes the maximum sensitivity of course. Since U&lt;W, even if the fiber is slightly displaced left or right, all the beams still enter the light receiving region of the PD and the PD enjoys the maximum sensitivity. Namely, since the diameter U of the beam waist is smaller than the diameter W of the light receiving region (U&lt;W), the PD has a broad maximum sensitivity range from 0.8 mm to 1.3 mm.
Why does the dependence on the distance Z of the distortion IMD.sub.2 differ from the dependence on the distance Z of the sensitivity? The IMD.sub.2 does not peak at the center Z.sub.0 (=1.1 mm) of the maximum sensitivity range but rather peaks at a farther point (1.2 mm). The previous invention succeeded in satisfying both requirements of the sensitivity and the distortion by making the best use of the asymmetry of the distortion and the sensitivity. Then why does the asymmetry occur between the sensitivity and the distortion? If a lens were to be free from aberration, the beams emitted from a light source should converge to an image point (beam waist) with a certain aperture and should diverge from the image point with the same aperture. Then the beams should be entirely symmetric both in the front direction (increasing Z) and in the rear direction (decreasing Z) with regard to the image point (beam waist), if the lens were immune from aberration. Then the distortion should also be symmetric both in the front direction and the rear direction with regard to the image point like the aperture.
However, lenses have aberration. In particular, a ball lens has a large aberration. Due to the aberration, the beams refracted by a lens cross the central axis line not at a common point but at different points distributed continually along the center axis line. The crossing points vary as a function of the radial distances of beams from the lens axis. A lens with aberration refracts far-axis beams stronger than near-axis beams. Far-axis beams cross the lens axial line earlier than near-axis beams. In short, far-axis beams converge more swiftly than near-axis beams. The farthest limit of the crossing points of near-axis beams is called "Gauss' image" point. Far-axis beams cross the axis at points closer to the lens than Gaussian image. The fiber.multidot.lens distance Z has a reciprocal relation to the lens.multidot.image distance Y which is simply denoted by the lens formula. The longer the fiber.multidot.lens distance Z becomes, the shorter the lens.multidot.image distance Y. The distortion of signal must originate from excess concentration of beams. Localized, concentrated power must induce the distortion of signals. Near-axis beams have a larger power than that of far-axis beams. Thus, the power density is the highest at Gauss' image point along the axis, because near-axis beams meet with the axis there. The Inventors suppose that when the light receiving surface of a PD happens to coincide with the Gaussian image point, the distortion is maximum, because such a disposition forces light power to concentrate at a narrow spot on the PD surface. Since dense light power hits at a narrow spot of the pn-junction, the second order or higher order effect appears on the photocurrent. The appearance of harmonics induces the intervention between different frequencies and enhances the distortion of electric signals.
If the fiber end is further pulled forward from the Gaussian image point, all the beams diverge. The beam diameter U is wider than the diameter W of the PD light receiving region (U&gt;W). Sensitivity starts to reduce. On the contrary, when the fiber end is pushed backward toward the lens from the Gaussian image point, far-axis beams begin to converge just on the light receiving surface of the PD. The beam diameter U is smaller than the diameter W of the PD receiving region (U&lt;W). The sensitivity maintains the maximum level for a while. The sensitivity curve remains flat at the maximum. When the fiber end is further advanced toward the lens, the beam diameter U becomes bigger than the PD receiving surface diameter W (U&gt;W). Sensitivity starts to degrade at the point of U=W close to the lens. The sensitivity therefore keeps the same maximum for the fiber end positions from the lens-close point of U=W to the Gaussian image point. But the peak of distortion exists at the Gaussian image point. The asymmetry between the sensitivity and the distortion is perhaps caused by the discrepancy of the Gaussian image point from the middle point of the maximum sensitivity range. The forward deviation of the Gaussian image point is caused by the aberration of the lens. A ball lens has the strongest aberration among spherical convex lenses. The asymmetry between the sensitivity and the distortion therefore appears conspicuously in the PD device relying upon a ball lens. The Inventors once discovered a new PD.multidot.lens.multidot.fiber disposition which allows the PD module to suppress the distortion down to a value less than -75 dBc and to maintain the maximum sensitivity. The PD module still had axially aligning PD, lens and fiber end. Three optical parts aligned on Z-axis in the previous module. However, the fiber end approached farther to the lens. The lens.multidot.fiber distance Z was determined to be smaller than the middle point Z.sub.0 of the maximum sensitivity range in FIG. 4. FIG. 4 shows a fall of the distortion in the region nearer to the lens than Z.sub.0. Namely, the fiber end did not form an image on the PD plane but formed the image on another plane which is farther from the lens than the PD in the improved module. The PD is placed at a non-focus point. Then the previous module may be named a "defocus type" module and the principle supporting the PD module may be called a defocus method. The axial defocus (Z&lt;Z.sub.0) alleviated the distortion without degrading the sensitivity.
At the beginning of optical CATV, a small number of channels could satisfy demand. The present optical CATV is usually provided with 40 channels for transmitting signals as a standard. Future optical CATV will further require the system of transmitting signals of 80-110 channels. The requirement for the number of channels is increasing. The reception frequency band of receiving sets has been 450 MHz till now. However, the increase of the channel number will demand 860 MHz for the frequency band of receiving sets instead of 450 MHz. Thus, the frequency band must be doubled in the optical CATV in the near future. The development of CATV increases the number of subscribers. The area of transmission is further widened. The broadcasting station uses high power laser diodes (LDs) for sending signals of many channels to a great number of houses of subscribers. Since high power lasers emit strong beams, more than 1 mW of light power sometimes shoots the PDs of some subscribers which are close to the station. The distortion is severe in the PD modules, since high power light and a great number of channels emphasize the distortion of signals. High power light from strong lasers may alleviate the need to enhance sensitivity but intensifies the need to lower distortion. This tendency for more channels and wider area for optical CATV systems makes it necessary to reduce the distortion of PD modules at the subscriber end. Further, prevalence of optical CATVs requires more inexpensive and more sophisticated PD modules. It is a matter of urgency to produce low-distortion, high sensitivity and stable photodiode modules.